Business and finance: How much are estimated monthly variable costs using the high-low method?

Rooter’s Cleaning Services provided data concerning the costs incurred to clean hotel rooms for which hotel customers pay $150 per night. Data for the past 7 months are as follows:

                January      February      March     April     May      June      July

Number of rooms cleaned 250           160             200       150        270        170        260
Cleaning cost                 $6,450         $4,060      $5,100    $4,100    ,000    $4,200    $6,530
How much are estimated monthly variable costs using the high-low method?

How much are estimated monthly variable costs using the high-low method?